N-color scalable LED driver

ABSTRACT

Light-emitting diode (LED) driver systems that are scalable and can be used for N-color LED systems are provided. An LED system having N LED strings of different color can be efficiently driven with independently controllable constant current sources for each string from a single power source. The driver system can include a power converter having an inductor.

CROSS REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of U.S. patent application Ser. No.15/504,561, filed Feb. 16, 2017, which is a national stage patent filingof International Patent Application No. PCT/US2015/048595, filed Sep. 4,2017, which claims the benefit of U.S. Provisional Application Nos.62/046,108, filed Sep. 4, 2014, which are incorporated by reference asif disclosed herein in their entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under NSF Contract No.EEC-0812056 and support under NYSTAR Contract No. C090145. Thegovernment has certain rights in the invention.

BACKGROUND

Light-emitting diodes (LEDs) of different color have different forwardvoltages as they require different excitation to produce light output.Therefore, LED strings of different color can't be driven with a singledirect current (DC) or alternating current (AC) bus voltage. Also, dueto the non-linear I-V (current-voltage) characteristics and negativetemperature coefficient, LEDs have to be driven with constant currentsources.

Traditionally, LED lighting systems use either single- ormultiple-string phosphor-converted white LEDs to deliver the desiredlumen level. The phosphor-converted white LED lighting systems withmultiple LED strings may have different forward voltages due totemperature variations, non-linear characteristics of LEDs, and poorbinning in LED manufacturing. LED drivers that drive multiple LEDstrings from a single voltage or current source require currentbalancing mechanisms for constant current regulation in each LED string.Passive and active approaches have been used to counteract voltagemismatch in LED strings and to provide constant currents.

The passive approaches are based on using passive elements, such ascapacitors, transformers, and inductors, in different configurations.Capacitor-based current balancing mechanisms underutilize the LEDs andalso depend on the tolerance of the capacitances used. Transformer andinductor passive approaches are bulkier in size.

The active balancing approaches are either based on using conventionallinear current regulators, such as current mirrors, or by usingindividual power converters to drive each of the LED strings. In thecase of linear current regulators, the efficiency of the converterdecreases with the increase in the voltage mismatch between the LEDstrings. Individual switched mode regulators providing constant currentto each LED string are more efficient but are not cost effective due totheir higher component count.

BRIEF SUMMARY

Embodiments of the subject invention provide advantageous light-emittingdiode (LED) driver systems, as well as methods of fabricating the same,methods of using the same to drive LED systems, and control schememethods for controlling LED systems. Systems of the subject inventionare scalable and can be used for N-color LED systems (where N is aninteger such as an integer greater than one). An LED system having N LEDstrings, wherein at least two of the strings are for an LED of adifferent color than each other, can be efficiently driven withindependently controllable constant current sources for each string froma single power source. For example, all LED strings can be of adifferent color than all other LED strings. The driver system caninclude a power converter, such as an AC-DC converter or a DC-DCconverter. The power converter can include at least one inductor, and inan embodiment, the power converter can include exactly one inductor.Systems of the subject invention can allow for highly efficient,high-power-density multi-color LED systems to be designed at areasonable cost.

In an embodiment, an N-color scalable LED driving system can include: apower converter comprising an inductor and configured to provide a DCoutput; a control circuit configured to receive input signals andprovide output control signals to the power converter; and a loadconnected to the power converter and receiving the DC output from thepower converter. The load can include a first LED string for a first LEDof a first color and a second LED string for a second LED of a secondcolor different from the first color. The first LED can be ahigher-voltage LED than is the second LED, and the second LED string caninclude a switch in series with the second LED.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a block diagram of a light-emitting diode (LED) driversystem according to an embodiment of the subject invention.

FIG. 1B shows a block diagram of an LED driver system according to anembodiment of the subject invention.

FIG. 2 shows a diagram of a control scheme according to an embodiment ofthe subject invention.

FIG. 3 shows a diagram of a control scheme according to an embodiment ofthe subject invention.

FIG. 4 shows a block diagram of an LED driver system according to anembodiment of the subject invention.

FIG. 5 shows a block diagram of an LED driver system according to anembodiment of the subject invention.

FIG. 6 shows a circuit diagram of an LED driver system according to anembodiment of the subject invention.

FIG. 7 shows steady state waveforms of inductor current and switchcontrol signals for a system according to an embodiment of the subjectinvention.

FIG. 8 shows waveforms of inductor current and control logic signals fora system according to an embodiment of the subject invention.

FIG. 9 shows waveforms of modulator controller voltages for a systemaccording to an embodiment of the subject invention.

FIG. 10 shows waveforms of diode currents and node voltage for a systemaccording to an embodiment of the subject invention.

FIG. 11 shows waveforms of LED string voltages for a system according toan embodiment of the subject invention.

FIG. 12 shows waveforms of LED string currents for a system according toan embodiment of the subject invention.

FIG. 13 shows waveforms of inductor current and control signals for asystem according to an embodiment of the subject invention.

DETAILED DESCRIPTION

Embodiments of the subject invention provide advantageous light-emittingdiode (LED) driver systems, as well as methods of fabricating the same,methods of using the same to drive LED systems, and control schememethods for controlling LED systems. Systems of the subject inventionare scalable and can be used for N-color LED systems (where N is aninteger such as an integer greater than one). An LED system having N LEDstrings, wherein at least two of the strings are for an LED of adifferent color than each other, can be efficiently driven withindependently controllable constant current sources for each string froma single power source. For example, all LED strings can be of adifferent color than all other LED strings. The driver system caninclude a power converter, such as an AC-DC converter or a DC-DCconverter. The power converter can include at least one inductor, and inan embodiment, the power converter can include exactly one inductor.Systems of the subject invention can allow for highly efficient,high-power-density multi-color LED systems to be designed at areasonable cost. Due to its simplicity, the entire control scheme issuitable for integrated circuit (IC) implementation.

Similar to multi-string phosphor-converted white LED lighting systems,multi-color LED lighting systems also have different forward voltages,as they require different excitation energy levels for the generation ofdifferent color light. Unlike multi-color LED systems, thephosphor-converted white LED systems cannot be controlled dynamically toprovide a desired color temperature (CT). Therefore, multi-color LEDsystems are preferable to phosphor-converted white LED lighting systemsfor future lighting applications. Future LED drivers for these systemsshould be capable of providing constant current to each color LED stringand should have the ability to independently control the current in eachstring through dimming techniques, such as pulse-width modulation (PWM)dimming and/or analog dimming, to achieve a desired color point in a CIE(International Commission on Illumination) chromaticity diagram.Moreover, the LED driver should be scalable and modular with simplecontrol. Techniques that can provide both current balancing andindependent current control can be used to realize multi-color LEDdrivers. Embodiments of the subject invention provide active currentbalancing schemes that can result in high-power-density multi-color LEDsystems using simple, modular, and scalable control schemes toindependently control currents of N strings (where N is an integer, suchas an integer greater than one). This can be accomplished using a singlepower converter.

N-color LED strings (where N is an integer, such as an integer greaterthan one) inherently have different string voltages. Typically, whenconnected in parallel to a single current source/voltage source, onlythe string with the lowest voltage will produce light output.Embodiments of the subject invention provide for a constant currentsource equivalent to the sum of currents desired by N-color LED stringsto be generated from a single source. An alternating current-directcurrent (AC-DC) or direct current-direct current (DC-DC) power convertercan be used and can operate at a switching frequency (f_(s)).

The N-color LED strings can be connected in parallel as a load to theAC-DC or DC-DC power converter and can be supplied by a single currentsource. If no control is present, then only the string with the lowestvoltage would produce light output. However, at least one of the LEDstrings can have a switch. In an embodiment, each color LED string,except the string with the highest voltage, can have a switch. Theseswitches can be in series with the LED strings, respectively. Theseswitches (e.g., switches in quantity of N−1, where N is the number ofLED strings) can be switched ON/OFF (or potentially not switched,depending on the circumstances) in each switching cycle. This can bedone through the use of a novel and advantageous modular feedbackcontrol scheme, which can lead to providing N constant current sources.

In many embodiments, a control scheme or method used to control theswitches can include measuring individual switch currents of theswitches (e.g., a quantity of N−1 switches) and integrating these duringevery switching cycle (T_(s)). A switching cycle can be, for example,the inverse of the switching frequency (T_(s)=1/f_(s)). The integratedswitch currents, which can be equal to the average current in an LEDstring, can be compared with the desired constant current (i.e., areference value). This comparison can be accomplished using, e.g., acomparator. Based on the comparison (e.g., the output(s) of thecomparator), the switches (e.g., a quantity of N−1 switches) can beswitched ON/OFF (on or off; or potentially not switched, depending onthe circumstances) to produce a constant current in each string. Byvarying the reference current with dimming techniques, such aspulse-width modulation (PWM) diming and/or analog dimming, the currentin each string can be controlled independently.

In an embodiment, the control scheme can be such that all switches(e.g., a quantity of N−1 switches) are initially kept ON (i.e., closed)at the beginning of each switching cycle (TO. Even with all switcheskept in the ON state, the current will only flow in the LED string withthe lowest voltage, and the currents in the other strings will be zero(or approximately zero). Once the integrated current in the lowestvoltage string reaches a desired reference value (e.g., a predeterminedreference value), the switch in the respective LED string can beswitched OFF (i.e., open). This sequence can be followed for allswitches present (e.g., all LED strings except for the string with thehighest voltage, which can be a quantity of N−1 LED strings). Thus, adesired average current can be provided to LED strings (e.g., all LEDstrings except for the string with the highest voltage, which can be aquantity of N−1 LED strings). The power converter can produce a constantcurrent source equal to the sum of the desired currents in all strings(e.g., N strings). As the currents in the strings other than that withthe highest voltage (e.g., a quantity of N−1 LED strings) can be beingregulated individually as discussed, the current of the string with thehighest voltage (e.g., the N.sup.th string) can also be regulated to adesired value. This value can be, for example, equal to the differencebetween the power-converter-generated constant current and the sum ofaverage currents in the other LED strings (e.g., N−1 LED strings). Thecontrol scheme can therefore exploit the disadvantage of the N-color LEDstrings having different string voltages in developing a simple,modular, and scalable control scheme to drive N-color LED systems.

FIG. 1A shows a block diagram of an LED driver system according to anembodiment of the subject invention. The system can be a scalable LEDdriver system and can be used for LED strings having different colors(i.e., it can be an N-color scalable LED driver system, where N is aninteger such as an integer greater than one). Referring to FIG. 1A, thecontrol scheme can control the power converter to produce a constantcurrent (i_(L)). N-color LED strings with forward voltages {v₁, v₂ . . .v_(N)} can act as a load to the converter. The multi-color LED stringscan be arranged such that v₁<v₂< . . . <v_(N). If these multi-colorstrings were connected to a single voltage source or current sourcewithout any control, then only the LED string with the lowest voltagewould produce light output. These strings are controlled, though, by thecontrol scheme in the driver system of the subject invention.

In an embodiment, independent current control for each color LED stringcan be achieved by connecting switches (e.g., a quantity of N−1switches, where N is the total number of LED strings) {S₁, S₂ . . .S_(N-1)} in series with the strings, respectively (e.g., a quantity ofN−1 strings). The system can be such that the string with the highestvoltage (v_(N)) does not have a series switch. The switches {S₁, S₂ . .. S_(N-1)} can be placed in different positions, as long as each switchpresent is in series with an LED string. For example, FIG. 1B shows ablock diagram of an LED driver system in which the switches are providedat a different position than they are in FIG. 1A. It is also possible tohave a switch in different LED strings in positions within the stringthat are different from each other (e.g., some switches in a positionsimilar to that shown in FIG. 1A and other in a position similar to thatshown in FIG. 1B). Also, the switch positions shown in FIGS. 1A and 1Bare for demonstrative purposes only; switches can be in other positionswithin the LED strings, as long as they are in series with the strings,respectively.

FIG. 2 shows a diagram of a control scheme implementation with PWMdimming. The control scheme can be used for controlling the powerconverter operating at switching frequency f_(s) to realize a constantcurrent i_(L) equal to the sum of the desired currents in N strings.Referring to FIG. 2, each string current control can be achieved byusing N−1 modular controllers. Illumination control signals {DIM₁, DIM₂. . . DIM_(N)} can be used as inputs to the modular controller.Illumination control signals {DIM₁, DIM₂ . . . DIM_(N)} in an adaptiveor dynamic lighting system can be obtained from, for example, ambientlight or color sensors, and can be dependent on the control algorithmused. Using the illumination control signals, the control scheme cangenerate the control signals {v_(C1), v_(C2) . . . v_(CN)} for differentdimming techniques. The summation of the control signals {v_(C1), v_(C2). . . v_(CN)} can provide the desired reference value to control thepower converter current (i_(L)). The reference for controlling thecurrent (i_(L)) can be derived from illumination control signals {DIM₁ .. . DIM_(N-1), DIM_(N)} based on the desired illumination levels foreach color LED string. Illumination control can be performed by, forexample, PWM dimming or analog dimming. FIG. 3 shows a diagram of acontrol scheme implementation for generation of control signals {v_(C1),v_(C2) . . . v_(CN)} with analog dimming.

FIG. 4 shows a block diagram of an N-color LED driver system with anAC-DC power converter, according to an embodiment of the subjectinvention. This can be used when the input is AC. FIG. 5 shows a blockdiagram of an N-color LED driver system with a DC-DC power converter,according to an embodiment of the subject invention. This can be usedwhen the input is DC. It is important to note that the power convertertopologies shown in FIGS. 4 and 5 are for demonstrative purposes only.The AC-DC converter (for AC input) or DC-DC converter (for DC input) canbe a converter of any topology that can provide a constant current.

FIG. 6 shows a block diagram of an LED driver system for three LEDstrings of different colors (N=3). This system uses PWM dimming, thoughanalog dimming could instead be used. The power converter topology usedis a buck converter, though this is for demonstrative purposes only. Thepower converter when controlled with an average current mode control (asshown in FIG. 6) can produce a constant current. The current produced bythe power converter can be shared among the three (N) strings using two(N−1) modular controllers. These controllers can monitor the currents{i_(S1), i_(S2)} flowing through the switches {S₁, S₂}, and they can beintegrated to produce v_(X1), v_(X2). These voltages v_(X1), v_(X2) canbe compared with the control signals {v_(C1), v_(C2)} to generatesignals {G₁, G₂} for sharing the current i_(L) among two (N−1) strings(e.g., the strings other than that with the highest voltage). In steadystate, the integrated switch current can be equal to the average currentin the LED string, as the average current in the capacitors C₁, C₂=0. Atthe beginning of each switching cycle (T_(s)), the switches {S₁, S₂} canbe kept ON. This can be achieved with, for example, a simple logic byutilizing the power converter control signal ‘G’ as a clock to two (N−1)modular controller D-Flip Flops. Even though the switches {S₁, S₂} arekept ON at the beginning of the switching cycle, the current (i_(S1))will only flow in the LED string with the lowest voltage (v₁), and thecurrents (i_(S2), i_(S3)) in the other strings will be zero (orapproximately zero) at the very beginning of the cycle, as v₁<v₂<v₃,which makes the diodes {D₂, D₃} reverse-biased. Once v_(X1) reaches thereference value v_(C1), the switch S₁ in that string can be switched OFFand the current can start flowing in that string with voltage v₂<v₃.This process can continue thorough all strings. Therefore, if theaverage current in two (N−1) strings is controlled, then the averagecurrent in the third (N^(th)) string (e.g., that with the highestvoltage) can also be regulated to the desired value as defined by the v₀(v_(CN)). This is because i_(L) is governed by v_(C1)+v_(C2)+v_(C3)(Σv_(CN)). In the system shown in FIG. 6, the control scheme has beenimplemented using mixed circuits (analog+digital). However, this is fordemonstrative purposes only and the entire control can also beimplemented using only digital controllers, such as microcontrollers anddigital controllers, or only analog controllers.

Although FIG. 6 shows an LED driver system for driving three LEDstrings, this is for demonstrative purposes only. Systems and methods ofthe subject invention can be used to drive any number of LED strings,each of which can be for a different color LED. For example, N can beany of the following values, at least any of the following values, or atmost any of the following values: 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12,13, 14, 15, 16, 17, 18, 19, or 20. N is preferably at least two.

In an embodiment, a method of fabricating an N-color LED driver systemcan include providing the components and assembling them to give asystem as described herein.

In another embodiment, a method of driving an LED system having Nstrings of different color can include providing an N-color LED driversystem as described herein and using the driver system for its intendedpurpose. The driver system can drive the LED system using a controlscheme as described herein.

In another embodiment, a method of controlling an LED system having Nstrings of different color can include implementing a control scheme asdescribed herein. The method can also include using an LED driver systemas described herein to implement the control scheme.

Systems and methods of the subject invention allow for highly efficient,high-power-density multi-color LED systems to be designed at areasonable cost. Due to the simplicity of the control schemes of thesubject invention, the systems are suitable for IC implementation.

Systems and methods of the subject invention can also advantageously beused in smart lighting systems. Smart lighting systems that are adaptivein nature require high-quality, tunable white light with differentcorrelated CTs, along with a black body curve and color-tunable light.High quality tunable white light is desired in general lighting toimplement technologies like daylight harvesting in smart buildings.Color tunable light can be used for many applications, including accentlighting and plant growth lighting. Therefore, multi-color LED lightingsystems with N-color LED strings are required for adaptive lighting.These N-color LED strings fundamentally have different forward voltages,as discussed herein, and the systems and methods of the subjectinvention can advantageously drive these multi-color LED lightingsystems efficiently and at low cost.

Systems and methods of the subject invention can also be used fordriving liquid crystal display (LCD) backlighting, organic LEDs (OLEDs),solar-powered LED lighting systems, driving and power on Ethernet (PoE)LED systems. In addition, they can be used for charging of batteries,fuel cells, super capacitors, or combinations of these devices that havedifferent voltages, by using a single power source and a singleinductor.

The systems, methods, and processes described herein can be embodied ascode and/or data. The software code and data described herein can bestored on one or more computer-readable media, which may include anydevice or medium that can store code and/or data for use by a computersystem. When a computer system reads and executes the code and/or datastored on a computer-readable medium, the computer system performs themethods and processes embodied as data structures and code stored withinthe computer-readable storage medium.

It should be appreciated by those skilled in the art thatcomputer-readable media include removable and non-removablestructures/devices that can be used for storage of information, such ascomputer-readable instructions, data structures, program modules, andother data used by a computing system/environment. A computer-readablemedium includes, but is not limited to, volatile memory such as randomaccess memories (RAM, DRAM, SRAM); and non-volatile memory such as flashmemory, various read-only-memories (ROM, PROM, EPROM, EEPROM), magneticand ferromagnetic/ferroelectric memories (MRAM, FeRAM), and magnetic andoptical storage devices (hard drives, magnetic tape, CDs, DVDs); networkdevices; or other media now known or later developed that is capable ofstoring computer-readable information/data. Computer-readable mediashould not be construed or interpreted to include any propagatingsignals.

A greater understanding of the present invention and of its manyadvantages may be had from the following examples, given by way ofillustration. The following examples are illustrative of some of themethods, applications, embodiments and variants of the presentinvention. They are, of course, not to be considered as limiting theinvention. Numerous changes and modifications can be made with respectto the invention.

EXAMPLE 1

An LED driver system was fabricated for driving three LED strings ofdifferent colors (N=3). The system fabricated is shown in the blockdiagram of FIG. 6. The system was set up to use PWM dimming, and thepower converter topology used was a buck converter. The current producedby the power converter was shared among the three (N) strings using two(N−1) modular controllers. These controllers monitored the currents{i_(S1), i_(S2)} flowing through the switches {S₁, S₂}, and they wereintegrated to produce v_(X1), v_(X2). These voltages v_(X1), v_(X2) werecompared with the control signals {v_(C1), v_(C2)} to generate signals{G₁, G₂} for sharing the current i_(L) among two (N−1) strings (thestrings other than that with the highest voltage). In steady state, theintegrated switch current was equal to the average current in the LEDstring, as the average current in the capacitors C₁, C₂=0. At thebeginning of each switching cycle (T_(s)), the switches {S₁, S₂} werekept ON. This was achieved with a simple logic by utilizing the powerconverter control signal ‘G’ as a clock to two (N−1) modular controllerD-Flip Flops. Even though the switches {S₁, S₂} were kept ON at thebeginning of the switching cycle, the current (i_(S1)) only flowed inthe LED string with the lowest voltage (v₁), and the currents (i_(S2),i_(S3)) in the other strings were zero (or approximately zero) at thevery beginning of the cycle, as v₁<v₂<v₃, which made the diodes {D₂, D₃}reverse-biased. Once v_(X1) reached the reference value v_(C1), theswitch S₁ in that string was switched OFF and the current startedflowing in that string with voltage v₂<v₃. This process was repeated inthe second string. After the average current in two (N−1) strings wascontrolled, then the average current in the third (N^(th)) string (thatwith the highest voltage) was also regulated to the desired value asdefined by the v.sub.C3 (v_(CN)). This was because i_(L) was governed byv_(C1)+v_(C2)+v_(C3) (Σv_(CN)). As seen in FIG. 6, the control schemewas implemented using mixed circuits (analog+digital).

This (N=3)-color LED driver system was tested to determine parameters ofthe system. FIG. 7 shows the steady state waveforms for this system.FIG. 8 shows experimental waveforms of inductor current (i_(L)) andcontrol logic signals G₁, G₂, and G for S₁, S₂, and S, respectively.FIG. 9 shows experimental waveforms of modulator controller voltagesv_(X1), V with respect to the control signals G₁ and G₂. FIG. 10 showsexperimental waveforms of the node voltage v_(y) and of the currentsi_(S1), i_(S2), i_(S3) in the diodes D₁, D₂, D₃, respectively. FIG. 11shows experimental waveforms of the LED string voltages v₁, v₂, v₃. FIG.12 shows experimental waveforms of the LED string currents i₁, i₂, i₃.FIG. 13 shows experimental waveforms of inductor current (i_(L)) ofcontrol signals v_(p1), v_(p2), v_(p3) to strings 1, 2, and 3 with 33%,66% and 100% PWM dimming, respectively.

Although the disclosed subject matter has been described and illustratedwith respect to embodiments thereof, it should be understood by thoseskilled in the art that features of the disclosed embodiments can becombined, rearranged, etc., to produce additional embodiments within thescope of the invention, and that various other changes, omissions, andadditions may be made therein and thereto, without parting from thespirit and scope of the present invention.

What is claimed is:
 1. A light-emitting diode (LED) driving system,comprising: a power converter comprising an inductor and configured toprovide a direct current (DC) output; a control circuit configured toreceive input signals and provide output control signals to the powerconverter; and a load connected to the power converter and receiving theDC output from the power converter, wherein the load comprises: a firstLED string for a first LED of a first color; and a second LED string fora second LED of a second color different from the first color, whereinthe first LED is a higher-voltage LED than is the second LED, andwherein the second LED string comprises a first switch in series withthe second LED; and wherein the controller is configured to implement acontrol scheme comprising the following steps: i) setting all switchesin the strings ON; ii) measuring individual switch currents for allswitches in the strings; iii) integrating these switch currents; iv)once an integrated current in the LED string that has the lowest-voltageLED of all present current strings that have a switch that is ON reachesa predetermined reference value for that LED string, switching theswitch in that LED string to OFF; v) repeating step iv) until all seriesswitches in the strings are OFF.
 2. The LED driving system according toclaim 1, wherein the load further comprises at least one additional LEDstring, wherein each additional LED string includes an LED of a colorthat is different from that of the LED of all other additional LEDstrings and also different from the first and second colors, wherein theLED of each additional LED string is a lower-voltage LED than is thefirst LED, and wherein each additional LED string further includes aswitch in series with the LED of each additional LED string.
 3. The LEDdriving system according to claim 1, wherein the predetermined referencevalue is set for each LED string individually.
 4. The LED driving systemaccording to claim 1, wherein the first LED string is regulated by thecontrol scheme to a predetermined reference value for the first LEDstring, and wherein the predetermined reference value for the first LEDstring is equal the difference between the power-converter-generatedconstant current and the sum of average currents in all other LEDstrings.
 5. The LED driving system according to claim 1, wherein thefirst LED string does not include a switch in series with the first LED.6. The LED driving system according to claim 1, wherein the controlscheme further comprises: vi) repeating steps i)-v) each switching cycle(T_(s)), wherein the switching cycle (T_(s)) is a length of time equalto the inverse of a switching frequency (f_(s)) of the power converter.7. The LED driving system according to claim 1, wherein the inputsignals received by the controller are illumination control signalswherein the illumination control signals are obtained from ambientlight, at least one color sensor, or both, wherein a reference value forthe power-converter-generated constant current is derived from theillumination control signals based on desired illumination levels foreach LED string present, and wherein illumination control within thecontroller is performed by pulse-width modulation (PWM) dimming oranalog dimming.
 8. The LED driving system according to claim 1, whereina reference value for the power-converter-generated constant current isbased on a summation of the output control signals from the controller.9. The LED driving system according to claim 1, wherein the load isconnected in parallel with the power converter.
 10. The LED drivingsystem according to claim 1, wherein the power converter is analternating current-DC (AC-DC) power converter configured to receive anAC input.
 11. The LED driving system according to claim 1, wherein thepower converter is a DC-DC power converter configured to receive a DCinput.
 12. The LED driving system according to claim 1, wherein thepower converter is a buck converter.
 13. The LED driving systemaccording to claim 1, wherein the power converter includes exactly oneinductor.